Nonequilibrium Behavior of the Dynamic Resistive Transition in Superconducting Thin Films

A detailed investigation of the dynamic resistive transition in superconducting thin films has been performed. The superconductors were excited into the nonequilibrium resistive state by picosecond optical excitation producing a "deep" excitation of the superconductor, or by picosecond risetime current pulse injection producing a "shallow" excitation of the superconductor. Under optical excitation, the superconducting lead and tin films underwent a first-order transition into the intermediate state when the critical optical power was exceeded. In the intermediate state, superconducting and normal regions coexist producing a resistance less than the normal state value. Only at much larger powers did the superconductor enter the normal state. The range of optical power producing the intermediate state was proportional to the strength of the electron-phonon interaction. The dynamic behavior was found to be in agreement with Elesin's theory of the intermediate state. Based upon Elesin's prediction of the superconducting-normal domain boundary motion, we developed a simple model to predict the dynamic resistive behavior. The model predicted the observed features of the resistance, thereby the intermediate state, both qualitatively and quantitatively. Under current injection, the superconducting-normal transition (SN) of tin films was found to be dominated by a delay-time. As a consequence, it was possible to exceed the dc critical current of the superconductor for a short period of time. The behavior was investigated by a transimission experiment that increased the effective bandwidth of our apparatus into the subnanosecond regime. The behavior of the SN transition was adequately predicted by TDGL theory, employing the inelastic electron-phonon scattering time as the fitting parameter. The subsequent normal-superconducting (NS) relaxation was found to be determined by the initial state excited by the current pulse. An anomalous relaxation, where the relaxation time varied inversely with the cube of the excitation amplitude, was measured. TDGL theory predicted the relaxation anomaly to be a result of the initial phase-slip state excited by the current pulse.